In the manufacture of fiber optic cables, two design types are most frequently employed that are generally referred to as "central-core" and "loose-tube" designs. In the central-core design, a number of optical fibers are contained within a central tube, which is located at the center of the fiber optic cable. Further, strength members are positioned between the central tube and an outer plastic jacket of the cable. In contrast, loose-tube fiber optic cables typically include a number of relatively small buffer tubes that are positioned around a central strength member, and each buffer tube encloses a number of optical fibers. The buffer tubes are longitudinally stranded around the central strength member, meaning that the buffer tubes are rotated around the central strength member along the length of the fiber optic cable. An example of such a loose-tube fiber optic cable is disclosed in U.S. Patent No. 5,621,841.
Fiber optic cables are commonly deployed by installing them in ducts, burying them in the ground, or suspending them between above-ground poles, or the like, along aerial rights of way. Because aerial rights of way have been widely established, fiber optic cables can often be more cost effectively deployed in aerial rights of way than in subterranean rights of way. Further, fiber optic cables that are buried in the ground are often damaged by unintentional excavation of the cables. Thus, it is common for fiber optic cables to be deployed in aerial rights of way.
It is common to deploy a fiber optic cable in an aerial right of way by suspending sections of the cable between spaced-apart poles, or the like. As a result, each section of the cable that is intermediate of a pair of the poles is supported solely by the poles and is constantly tensioned due to the weight of the cable. Such tensile loading is increased by wind impinging upon the cable or accumulations of ice and snow upon the cable. Tensile loading of a fiber optic cable can cause the optical fibers within the cable to become strained in a manner that disadvantageously causes attenuation of optical signals propagating there through.
It is conventional to avoid fiber strain in aerially deployed fiber optic cables by lashing the cables to aerially suspended metal wires or non-metallic rods, or by otherwise attaching the cables to those suspension wires and suspension rods, as described in U.S. Pat. No. 4,662,712. Whereas the metal suspension wires can alleviate strain in fiber optic cables, the metal suspension wires, as well as any metal contained in the fiber optic cables, can attract lightning, which can seriously damage fiber optic cables. Further, an added expenditure is required to obtain metal suspension wires and non-metallic suspension rods that are used in combination with aerially deployed fiber optic cables. In addition, the use of the suspension wires or rods disadvantageously introduces additional steps in the manufacture or installation of fiber optic cables.
As a result of the foregoing, it is common to deploy all-dielectric, self-supporting (ADSS) fiber optic cables (without extra suspension wires or rods) along aerial rights of way so that sections of the cables are supported solely by spaced-apart poles and hardware located solely at the poles, or the like. It is common for such ADSS fiber optic cables to be of the loose-tube type, and for them to be constructed so that the optical fibers therein are not strained at the maximum rated tensile load for the fiber optic cable. The optical fiber strain is typically avoided by not coupling the optical fibers within the buffer tubes to one another, having the interior diameter of the buffer tubes be sufficiently large so that the optical fibers are substantially free to move within the buffer tubes, and in some cases by causing the optical fibers to be longer than the buffer tubes. That is, based on conventional wisdom, ADSS fiber optic cables are commonly constructed so that they contain excess optical fiber length per unit length of their buffer tube, the optical fibers are loosely arranged within their buffer tube (that is, the optical fibers are not in a unitary ribbon-like arrangement), and the buffer tubes are sufficiently large in diameter so that when the cable is loaded to its maximum rated tensile load, the optical fibers therein are free to move and therefore are not strained, even through the cable is strained. It is common for such conventional ADSS fiber optic cables to have lay lengths of from approximately 76 millimeters to 168 millimeters. The lay length is the distance along the central strength member in which the buffer tubes make a complete revolution around the central strength member.
In response to demands for increased optical fiber count in fiber optic cables, optical fiber ribbons have been developed. Optical fiber ribbons are planar arrays of optical fibers that are bonded together as a unit. Optical fiber ribbons are advantageous because many ribbons can be stacked on top of each other within a small space. As stacks of optical fiber ribbons are used in fiber optic cables to increase optical fiber counts, constraints remain on the outer diameters of the cables. As a result, and due to the optical fibers within an optical fiber ribbon being restricted from moving relative to one another, it is common for the optical fibers of loose-tube fiber optic cables containing optical fiber ribbons to be strained when the cable is strained due to tensile loading. The optical fiber strain occurs because the optical fiber ribbons cannot move sufficiently within their buffer tubes to prevent straining of the optical fibers.
As mentioned above, it is conventional for ADSS fiber optic cables to be designed so that there is no strain imparted on the optical fibers therein when the cable is exposed to the maximum rated tensile load. As also mentioned above, it is typical for optical fibers of optical fiber ribbons of a loose-tube fiber optic cable to be strained by tensile loading of the cable. Therefore, there is a teaching away from using optical fiber ribbons in loose-tube ADSS fiber optic cables. Nonetheless, there is a need for ADSS fiber optic cables that contain optical fiber ribbons, due to increasing demands for high fiber counts.